It is so far known from the state of the art that an active matrix liquid crystal display (LCD) using a low temperature polycrystalline silicon thin film transistor (LTPS-TFT) generally provides better driving capability and higher degree of integration than a display adopting amorphous silicon thin film transistors (a-Si TFT) currently in wide use for monitors of notebook computers and desktop personal computers. Thanks to such an advantage, the active matrix LCDs tend to be more frequently adopted for a high resolution LCD device.
In the meantime, an active matrix OLED device has recently emerged as one of the most competitive next generation of display units, in which the brightness of light emitting elements is subject to the changes in the amount of current flowing through an organic thin film element, so most important in the active matrix OLED is to secure the uniformity in thin film transistors, for example, the uniformity in threshold voltage (Vth) and field effect mobility. This is because a uniform current flow in these picture elements can be achieved by compensation of the threshold voltage in TFT. However, it is known from the state of the art that it is very difficult to manufacture an LTPS TFT with such a desired uniformity in threshold voltage and field effect mobility, which is usually processed under a low temperature environment of less than about 450° C. Therefore, various solutions have been so far sought to ensure the uniformity in TFT, with accesses in the side of physical circuits, for instance, among others, by providing a compensation circuit to each picture element in an active matrix OLED panel.
The basic picture cell scheme in an active matrix OLED may be generally divided into two categories, that is to say, a voltage programming type of inputting picture data with voltage and a current programming type of inputting picture data with current.
FIG. 1 represents the structure of picture elements widely used in the conventional current programming type of active matrix OLED, and FIG. 2 represents a timing diagram in the picture element of FIG. 1.
Referring to FIGS. 1 and 2, it is shown that a prior-art current programming type of picture element is configured to have four TFTs T1 to T4 and a capacitor Cstg, provided that two TFTs T3 and T4 of the four TFTs have the substantially identical electrical characteristic. Of this circuit configuration in FIG. 1, TFTs T1 and T2 serve as a switch as in an active matrix LCD, the capacitor Cstg serves to store a data voltage corresponding to a programmed current, and TFT T4 serves to have the current corresponding to the data voltage stored in capacitor Cstg flow into the OLED.
In the basic structure of picture element as shown in FIG. 1, the relationship between the input data current and the output OLED current can be obtained from the following formula:IDATA=½×k3×(VGS−VDD−Vth—T3)2 IOLED=½×k4×(VGS−VDD−Vth—T4)2 
wherein k represents a current-voltage relation in a saturation area, that is, k=μ×Cox×W/L, in which μ represents a field effect mobility, Cox a capacitance of insulating layer, W a channel width, and L a channel length, respectively.
Provided that the electrical characteristics of TFTs T3 and T4 are the same to each other in each picture element, the current scaling ratio IOLED/IDATA may be equal to k4/k3. Therefore, even if a threshold voltage in TFT changes in a current programming type of picture element, it is allowed to output an OLED current only dependent upon the data current irrespectively of the threshold voltage provided that the adjacent two TFTs (for instance, T3 and T4) in each picture element have the substantially same electrical characteristics.
Accordingly, it is appreciated that in case where the OLED threshold voltage deteriorates as a panel operating time becomes longer, the aforementioned prior-art picture cell structure will have a disadvantage that it causes the deviation in OLED output current to occur owing to kink characteristic in TFT T4.